The process of offset printing is based on certain physicochemical surface phenomena. The stage which precedes the application of the ink to the paper consists of phenomena of attraction and repulsion between the wetting solution and the ink, this allows for the formation of an image on an aluminium plate which is then directly transferred to a rubber blanket cylinder, and from there to the paper. In order to obtain satisfactory results it is necessary that both the ink and the solution, which is supplied to the wetting system of the machine, possess certain determined physicochemical features.
The most widely used wetting solution consists of water, with certain additives providing the system with determined features, and one part, between 8 and 10%, of isopropyl alcohol.
The use of isopropyl alcohol in the wetting solution ensures an interfacial tension between the water and the ink which allows for the achievement of a lesser degree of emulsification of the water in the ink, as a result of which the ink printed onto the paper benefits from a higher level of saturation than can be achieved with any other wetting system.
The isopropyl alcohol, or isopropanol, provides the solution with other advantages, such as a greater viscosity allowing for a more uniform transference, a greater drying speed, and a lowering of the surface tension which ensures that the film of the solution on the plate is very thin, and that it has a lesser tendency to mechanical emulsification.
Independently of the advantages which the addition of isopropyl alcohol, to the solution used in the wetting system, brings to offset printing this product also presents certain important disadvantages, such as inflammability and toxicity, which make it dangerous, and which have given rise to a generalized tendency towards the elimination of this product, to the extent that its use is completely, or partially, prohibited in some countries.
The problem which is posed, therefore, is how to eliminate isopropyl alcohol from the solutions used for wetting and the formulation of additives, which has so far been attempted without success, that will allow for such an elimination while maintaining the water-ink interface, this being the principal problem.
The need to reduce the surface tension, in order to avoid mechanical emulsification up to levels of water-alcohol, obliges the use of group III surface-active agents, however a greater concentration of these surface-active agents leads to a decrease of the interface, due to the affinity between the fatty acids of the ink and the hydrocarbon chains of these substances; this, together with the current impossibility of forming stable colloids which reduce the surface tension and maintain a greater interface, is the reason why alcohol cannot be eliminated from the wetting systems.